1. Field of the Invention
This invention relates to a new eco-friendly heterogeneous solid catalyst for use in reactions such as alkylation, acylation, olegomerization, isomerization, hydration, dehydration, etherification, esterification, hydrocracking and nitration of organic compounds for expected shape selectivity in mesoporous range and its method of manufacture. The catalyst of the present investigation is in the field of sulphated zirconia and mesoporous molecular sieves.
2. Description of the Related Art
Zeolites are the important family of solid acid catalysts having tridimentional crystalline aluminosilicate network with channels and cavities of molecular dimensions. The system of these molecular sieves produces materials with very high surface area and pore volume, which are capable of adsorbing great amounts of organic molecules. There has been, however, an ever growing interest in expanding the pore sizes of molecular sieve materials from the micropore region to mesopore region in response to the increasing demands in both industrial and fundamental studies. Mesoporous zeotype materials are inherently poor stability and weakly acidic.
In 1992, researchers at Mobil Corporation discovered the M41S family of silicate/aluminosilicate mesoporous molecular sieves with exceptionally large uniform pore structure (Kresge, C. T.; et al. Nature 1992, 359, 710-712, Beck, J. S.; et al. J. Am. Chem. Soc. 1992, 114, 10834-10843). This has resulted in a world-wide resurgence in this area. The template agent used is no longer a single, solvated organic molecule or metal ion, but rather a self-assembled surfactant molecular arrays. The mesoporous material synthesised in the above method possesses highly regular arrays of uniform-sized channels whose diameters are in the range of 15-100 .ANG. depending on the templates used, the addition of auxiliary organic compounds, and the reaction parameters.
Synthesis of mesoporous materials have been reviewed by Tanav, P. T. and Pinnavaia, T. J. (Science, 267, 865-867) There are four general methods of preparation of mesoporous materials and any one could be used to synthesize them in the laboratory.
The first three routes are based on ionic mechanisms while the fourth employs neutral templates to prepare hexagonal mesoporous molecular sieves (HMS). The last (IVth) route is based on the self-assembly between neutral primary amine micelles (S.sup.o) and neutral inorganic precursor (I.sup.o). This neutral S.sup.o I.sup.o produces mesostructures with larger wall thickness and complementary textural mesoporosities vis-a-vis those materials produced by routes I to III. The thicker pore walls improve the thermal and hydrothermal stability of the mesoporous framework. The S.sup.o I.sup.o pathway also allows for the facile recovery of the template by simple solvent extraction.
From the above literature it can be concluded that, the preparation of mesoporous material using the neutral templating method provide a better approach to get mesoporous materials. But, they too have the disadvantage of having very low acidity compared with other solid acid catalysts particularly for reaction requiring high acidity. With the possibility to generate active sites inside of the channels and cavities of zeolites and zeotypes, a very unique type of shape selective catalyst may be produced and it can be visualised as a microreactor. Therefore, any modification which can promote the surface acidity along with their molecular sieving property of these catalysts will be highly desirable.
On the other hand, catalysts based on zirconia and other metal oxides showed very high acidity and activity when small amount of sulphate was treated with them. Hino and Arata (Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun., 1980, 851-852) have reported that sulphated zirconia is an acid 10.sup.4 times stronger than 100% sulphuric acid and with the Hammett acidity function--H.sub.0 =16, it is considered as the strongest halide-free solid superacid ever reported. The strong acidity makes it attractive as a catalyst in many organic reactions such as alkylation, acylation, isomerization, etherification, esterification, hydration, dehydration, olegomerization, hydrocracking, etc. However, these superacidic materials have not found many applications because of their low surface area and non-shape selective nature. Developing a process for super acidic modified metal oxide catalyst with a high surface area along with molecular sieving property is a challenging field to researchers. This will have applications in heavy oil cracking and pharmaceuticals involving bulky structures.
Different approaches to introduce (a) strong acidic centres into molecular sieve materials and (b) shape selectivity into sulphate promoted metal oxide catalysts revealed that by introducing the shape selectivity into the oxide materials the surface area increases considerably. However, the desired activity and selectivity could not be achieved in either case. Nor was it possible to provide the catalyst having desired activity by loading the superacids on the molecular sieve material.